The goal of the proposed research is to continue a series of studies of the long-term effects of chronic cannabis use in Costa Rican males. We have a unique sample of very chronic users who have consumed marihuana on a habitual basis for about 30 years. This sample is working class largely literate urban, and does not use other drugs except for alcohol and tobacco. A sample of non-users comparable in age, education, and alcohol and tobacco use is also available. Although the initial Costa Rican study (1973-1975) yielded little evidence of adverse effects, our recent follow- up (1985-1986) found that users performed more poorly than non-users on cognitive measures involving attention, short-term memory, and mental effort. These findings correspond to studies of work in which users were found to engage in employment requiring less mental effort. Despite these differences, there was little evidence for deterioration in cognitive skills in the users over the 12 year time interval separating the two studies. Consequently, the degree of chronicity required to establish these adverse effects is not clear. Similarly, more information is needed concerning the nature of the cognitive deficits and the influence of cannabinoid levels in the body on cognitive performance. The specific aims of the proposed research are a) to replicate the results; b) recruit a younger, less chronic cohort; c) administer new tasks that measure more precisely cognitive skills that may be impaired in chronic users; and d) complete assay studies to determine cannabinoid levels in the body. Since the second study is the only well-controlled demonstration of adverse cognitive effects in non-intoxicated cannabis users who don't use other drugs, additional research is clearly indicated. If the hypotheses of this study are confirmed, the results may indicate even more adverse effects for habitual North American users of cannabis in whom greater demands are made for effortful mental processing. Such a study can only be completed in a cross-cultural setting because the degree of chronicity and absence of polydrug use does not exist in North American users. The proposed research provides a unique opportunity to explore cognitive changes in chronic marihuana users.